This invention relates to hydraulic systems for controlling a plurality of actuators such as hydraulic cylinders which are found, for example, in earth moving equipment such as excavators and cranes. In such a system, it is conventional to provide a pilot operated control valve for each actuator which is controlled by a controller through a pilot hydraulic circuit. The control valve functions to supply hydraulic fluid to the actuator to control the speed and direction of operation of the actuator. In addition, the control valve for each actuator controls the flow of hydraulic fluid out of the actuator. It is also common to provide counterbalance valves or fixed restrictions to control overrunning loads.
In U.S. Pat. No. 4,201,052 and U.S. Pat. No. 4,480,527, having a common assignee with the present application, there is disclosed and claimed a hydraulic system for accurately controlling the position and speed of operation of the actuators; which system is simple and easy to make and maintain; which system is unaffected by change of load pressure of various portions of the system or other actuators served by the same source; which system may not use flow from the pressure source in the case of overrunning loads on the actuators; wherein the control valves may be mounted adjacent the actuator for preventing loss of control of the load in case of malfunction in the hydraulic lines to the actuator; wherein the valves which control flow out of the actuator function to control the velocity in the case of energy generating loads; wherein the valve that controls flow into the actuator controls the velocity in the case of energy absorbing loads; wherein the valve system for each actuator can be mounted on its respective actuator and incorporates means for preventing uncontrolled lowering of the load in case of pressure failure due to breaking of the lines to the actuator mounted valve system; wherein the timing of operation of the valve controlling flow into the actuator and out of the actuator can be designed to accommodate the specific nature of the particular load. In certain high inertial loads such as swing drives on an excavator which utilize rotary actuators, smooth stopping and starting of the load and accurate positioning of the load are very essential.
In U.S. Pat. No. 4,407,122, there is disclosed a hydraulic system of the type shown in the aforementioned U.S. Pat. Nos. 4,201,052 and 4,480,527 modified to provide for smooth stopping and starting and accurate positioning of the load under inertial loads. The supply pressure out of the meter-in valve means is sensed and a force is applied to the meter-in valve means opposing the pilot pressure which tends to open the meter-in valve means.
Such a pressure mode system functions satisfactorily under pressure control for various functions of machines such as swinging or digging. However, under certain conditions, it is desirable to have the same machine function in either a true velocity or flow control mode. For example, if an earth moving machine or device is a front-end loader with a bucket to cut sod, where the system has pressure control, any sudden decrease of the load would cause the system to respond immediately and increase the flow to the hydraulic actuator or actuators and make the sod cutting operation precarious.
Accordingly, among the objectives of the invention are to provide a hydraulic system which can be operated selectively in a flow or velocity control mode or a pressure control mode; which can be readily applicable to a hydraulic system having velocity control; which in one form modulates force applied to the meter-in valve means opposing pilot pressure to the meter-in valve means; and which in another form can cause the hydraulic system to function either in a flow or velocity control mode or a pressure control mode.
In accordance with the invention, a hydraulic control system comprising a hydraulic actuator having opposed openings adapted to alternately function as inlets and outlets for moving the element of the actuator in opposite directions, a pump for supplying fluid to said actuator, a directional valve comprising pilot operated meter-in valves to which the fluid from the pump is supplied through first lines for controlling the direction of movement of the actuator, and a pilot operated meter-out valve associated with each opening of the actuator for controlling the flow out of said actuator. The pressure of fluid being supplied to the actuator by the meter-in valve is sensed and supplied to a line extending from the output of the meter-in valve. A valve is provided in this line to provide a force selectively on the meter-in valve opposing the movement of the meter-in valve by the pilot pressure. The valve may comprise either a modulating valve or a shut-off valve.